Semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor layer made of a wide bandgap semiconductor and including a gate trench; a gate insulating film formed on the gate trench; and a gate electrode embedded in the gate trench to be opposed to the semiconductor layer through the gate insulating film. The semiconductor layer includes a first conductivity type source region; a second conductivity type body region; a first conductivity type drift region; a second conductivity type first breakdown voltage holding region; a source trench passing through the first conductivity type source region and the second conductivity type body region from the front surface and reaching a drain region; and a second conductivity type second breakdown voltage region selectively formed on an edge portion of the source trench where the sidewall and the bottom wall thereof intersect with each other in a parallel region of the source trench.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. application Ser. No. 14/821,056, filed onAug. 7, 2015, and allowed on Nov. 30, 2016, which was a Continuation ofU.S. application Ser. No. 13/983,051, filed on Jul. 31, 2013, issued onSep. 15, 2015, as U.S. Pat. No. 9,136,322, which was a National Stageapplication of PCT/JP2012/052293, filed on Feb. 1, 2012. The prior USapplication, and international application, and the present continuationapplication claim the benefit of priority of Japanese applications2011-020730, filed on Feb. 2, 2011, and 2011-101786, filed on Apr. 28,2011. The disclosures of these prior US, international, and foreignapplications are incorporated herein by reference.

BACKGROUND ART

In general, a semiconductor power device mainly used for a system, suchas a motor control system or a power conversion system, in various powerelectronics fields is watched with interest.

As this type of semiconductor power device, an SiC semiconductor devicehaving a trench gate structure is proposed, for example.

For example, FIG. 1 of Patent Document 1 discloses a MOS semiconductordevice having an SiC substrate (1), an n-type high-resistance layer (2)formed on the SiC substrate (1), a p well layer (3) formed on the n-typehigh-resistance layer (2), an n⁺ emitter region (8) formed on a surfacelayer portion of the p well layer (3), a p⁺ contact region (12) passingthrough the n⁺ emitter region (8) and reaching the p well layer (3), atrench (5) passing through the p well layer (3) from a front surface ofthe n⁺ emitter region (8) and reaching the n-type high-resistance layer(2), a gate oxide film (6) formed on an inner surface of the trench (5),and a polysilicon gate electrode (7) embedded in the trench (5) (referto Patent Document 1, for example).

PRIOR ART Patent Document

-   Patent Document 1: Japanese Unexamined Patent Publication No.    2008-294210

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An electric field easily concentrates on a bottom portion of a gatetrench of a trench gate MOSFET, due to high potential difference betweena gate electrode and a drain electrode caused at a turnoff time. Thefield concentration serves as a factor in breakage of a gate insulatingfilm, and brings about reduction of the breakdown voltage of the MOSFET.

According to studies made by the inventors, it may be possible to relaxthis type of field concentration by implanting a p-type impurity intothe bottom portion of the gate trench.

However, a region where a channel is formed is exposed on an innersurface of the gate trench, and hence implantation of the p-typeimpurity into a channel region must be prevented when implanting thep-type impurity into the bottom portion of the gate trench. This isbecause it becomes difficult to control channel characteristics when thep-type impurity is implanted into the channel region.

An object of the present invention is to provide a semiconductor devicecapable of improving breakdown voltage resistance at an off time andfurther capable of controlling channel characteristics and a method forproducing the same.

Means for Solving the Problems

A semiconductor device according to the present invention for attainingthe aforementioned object includes a semiconductor layer made of a widebandgap semiconductor having a gate trench provided with a sidewall anda bottom wall, a gate insulating film formed on the sidewall and thebottom wall of the gate trench, and a gate electrode embedded in thegate trench to be opposed to the semiconductor layer through the gateinsulating film, while the semiconductor layer includes a firstconductivity type source region formed to be exposed on the side of afront surface of the semiconductor layer for partially forming thesidewall of the gate trench, a second conductivity type body regionformed on a side of the source region closer to a rear surface of thesemiconductor layer to be in contact with the source region forpartially forming the sidewall of the gate trench, a first conductivitytype drift region formed on a side of the body region closer to the rearsurface of the semiconductor layer to be in contact with the body regionfor forming the bottom wall of the gate trench, and a secondconductivity type first breakdown voltage holding region selectivelyformed on an edge portion of the gate trench where the sidewall and thebottom wall intersect with each other in a partial region of the gatetrench.

In order to attain the aforementioned object, the inventors have madedeep studies as to the mechanism of dielectric breakdown of a gateinsulating film at a turn-off time.

More specifically, when voltage rendering a drift region positive (+) isapplied between a source region and the drift region functioning as adrain (between a source and a drain) in a state where a semiconductordevice is off (i.e., a state where gate voltage is 0 V), an electricfield is applied to a gate insulating film interposed between a gateelectrode and the drift region. The electric field results from thepotential difference between the gate electrode and the drift region.Equipotential surfaces of extremely high potential are distributed on abottom portion of a gate trench on the basis of the gate electrode (0 V)and the interval between the equipotential surfaces is small, whereby anextremely large electric field is caused. This is such a mechanism that,when voltage approximate to device breakdown voltage is continuouslyapplied between the source and the drain, therefore, a portion of thegate insulating film present on the bottom portion of the gate trenchcannot withstand field concentration of the magnitude but causesdielectric breakdown.

According to the inventive semiconductor device, therefore, the firstbreakdown voltage holding region is formed on the edge portion of thegate trench in the semiconductor device in which the wide bandgapsemiconductor (whose bandgap Eg is not less than 2 eV, and preferably2.5 eV to 7 eV, for example) is employed, in such a vertical structurethat the source region and the drift region are vertically arrangedthrough the body region. Thus, a depletion layer resulting from junction(p-n junction) between the first breakdown voltage holding region andthe drift region can be generated in the vicinity of the edge portion ofthe gate trench. Equipotential surfaces of high potential based on thegate electrode can be kept at a distance from the gate insulating film,due to the presence of the depletion layer. Consequently, the electricfield applied to the gate insulating film can be diminished, wherebydielectric breakdown can be suppressed.

In the semiconductor device according to the present invention, further,the first breakdown voltage holding region is selectively formed on theedge portion of the gate trench on the partial region of the gatetrench. Therefore, no excess second conductivity type impurity isimplanted into the body region on a portion of the partial regionimmediately above the edge portion. Therefore, the concentration of thesecond conductivity type impurity in the body region can be maintainedas designed. Consequently, channel characteristics can be controlled.

Preferably, the impurity concentration in the first breakdown voltageholding region is higher than the impurity concentration in the driftregion. According to this structure, the depletion layer resulting fromthe junction (p-n junction) between the first breakdown voltage holdingregion and the drift region can be prevented from excessively spreadingin the semiconductor layer.

Preferably in the semiconductor device according to the presentinvention, the gate trench is formed in a latticed manner, thesemiconductor layer includes a plurality of unit cells partitioned bythe latticed gate trench and provided in the form of polygonal prismseach having a plurality of corner portions, and, in a case where eachunit cell has the source region, the body region and the drift region,the first breakdown voltage holding region is selectively formed on acorner edge portion of the gate trench formed on the corner portion ofthe unit cell.

In the case where the gate trench is formed in the latticed manner andthe unit cells provided in the form of polygonal prisms are arrayed onwindow portions of the latticed gate trench, dielectric breakdown of thegate insulating film particularly easily takes place in the vicinity ofthe corner edge portion of the gate trench formed on the corner portionof the unit cell. When the first breakdown voltage holding region isformed on the corner edge portion as in the semiconductor device of theaforementioned structure, therefore, dielectric breakdown of the gateinsulating film in the vicinity of the corner edge portion can beeffectively suppressed.

In the case where the first breakdown voltage holding region is formedon the corner edge portion of the gate trench, the first breakdownvoltage holding region may be formed to reach a portion of the bodyregion immediately above the corner edge portion.

In the unit cell provided in the form of a polygonal prism, voltageapplied to the gate electrode is so controlled that a channel is formedalong a side surface of the unit cell partially forming the sidewall ofthe gate trench. In other words, no channel is formed on the cornerportion of the unit cell, or the quantity of current flowing in such achannel is small even if the same is formed. Therefore, the breakdownpreventing effect for the gate insulating film can be further improvedwhile hardly influencing the performance of the device, by forming thefirst breakdown voltage holding region to reach the portion of the bodyregion immediately above the corner edge portion.

In the case where the first breakdown voltage holding region is formedon the corner edge portion of the gate trench, the first breakdownvoltage holding region may be selectively formed on an intersectionportion of the latticed gate trench.

Preferably, the semiconductor layer further includes a secondconductivity type second breakdown voltage holding region, formed on abottom wall of a linear portion of the latticed gate trench, having awidth narrower than the width of the linear portion.

Even if an electric field generated along the linear portion of the gatetrench acts on the gate insulating film, the electric field can berelaxed by a depletion layer resulting from junction (p-n junction)between the second breakdown voltage holding region and the driftregion, due to the structure. Consequently, the electric field generatedin the gate insulating film can be uniformly relaxed.

Further, the second breakdown voltage holding region is not formed on asidewall of the linear portion of the gate trench (i.e., a portion wherethe channel is formed in the unit cell), whereby reduction of theperformance of the device can also be prevented.

Preferably, the impurity concentration in the second breakdown voltageholding region is higher than the impurity concentration in the firstbreakdown voltage holding region. Preferably, the thickness of thesecond breakdown voltage holding region is smaller than the thickness ofthe first breakdown voltage holding region.

According to the structure, rise of channel resistance can besuppressed. The thicknesses of the first and second breakdown voltageholding regions denote thicknesses along a direction from the frontsurface toward the side of the rear surface of the semiconductor layer,for example.

Preferably in the semiconductor device according to the presentinvention, the semiconductor layer further includes a source trenchhaving a sidewall and a bottom wall, passing through the source regionand the body region from the front surface and reaching the driftregion, and a second conductivity type third breakdown voltage holdingregion selectively formed on an edge portion of the source trench wherethe sidewall and the bottom wall intersect with each other in a partialregion of the source trench.

According to the structure, a depletion layer resulting from junction(p-n junction) between the third breakdown voltage holding region andthe drift region can be generated in the vicinity of the edge portion ofthe source trench. Thus, aggregation of equipotential surfaces betweenthe edge portion of the gate trench and the edge portion of the sourcetrench can be prevented. Consequently, the electric field applied to thegate insulating film can be diminished, whereby dielectric breakdown canbe suppressed.

Preferably, the drift region includes a first region of a first impurityconcentration forming the bottom wall of the gate trench, and a secondregion of a second impurity concentration smaller than the firstimpurity concentration formed on a side of the first region closer tothe rear surface of the semiconductor layer to be in contact with thefirst region.

According to the structure, part or the whole of a portion of the driftregion in contact with the first breakdown voltage holding region can beturned into a region having the first impurity concentration relativelyhigher as compared with the remaining region (the second region) of thedrift region, by forming the first region on the bottom wall of the gatetrench. Thus, spreading of the depletion layer resulting from the p-njunction between the drift region and the first breakdown voltageholding region can be suppressed. Consequently, a path of electronsflowing in a channel (in the vicinity of the interface between the bodyregion and the gate insulating film) is not inhibited by the depletionlayer but a path of a sufficient magnitude can be ensured, wherebyincrease in on-resistance can be prevented.

Further, not the whole drift region is turned into thehigh-concentration first region but the relatively low-concentrationsecond region is formed on the side of the first region closer to therear surface of the semiconductor layer, whereby reduction of breakdownvoltage resulting from the formation of the first region can besuppressed.

The thickness of the first region may be greater than the thickness ofthe first breakdown voltage holding region, or may be not more than thethickness of the first breakdown voltage holding region.

When the thickness of the first region>the thickness of the firstbreakdown voltage holding region, it follows that the first regionentirely bears the junction between the drift region and the firstbreakdown voltage holding region, whereby spreading of the depletionlayer resulting from the p-n junction between the drift region (thefirst region) and the first breakdown voltage holding region can befurther suppressed.

When the thickness of the first region the thickness of the firstbreakdown voltage holding region, on the other hand, the implantationdepth of an impurity may be shallow in a case of forming the firstregion by ion implantation from the front surface of the semiconductorlayer, for example, whereby the first region can be easily formed withsmall implantation energy.

The thickness of the first region denotes a thickness along thedirection from the front surface toward the side of the rear surface ofthe semiconductor layer, for example.

The interface between the first region and the second region mayundulate in response to a step caused by partial lowering of the frontsurface of the semiconductor layer resulting from the formation of thegate trench, or may be at a constant distance from the front surface ofthe semiconductor layer.

Preferably, a portion of the gate insulating film on the bottom wall ofthe gate trench is thicker than a portion of the gate insulating film onthe sidewall of the gate trench, and a top portion thereof is below thedeepest portion of the body region.

Preferably, the front surface of the semiconductor layer is a C plane.

Preferably, the body region is formed by ion implantation.

A method for producing a semiconductor device according to the presentinvention includes the steps of forming a semiconductor layer, which isa semiconductor layer made of a wide bandgap semiconductor, including afirst conductivity type source region formed to be exposed on the sideof a front surface thereof, a second conductivity type body regionformed on a side of the source region closer to a rear surface to be incontact with the source region, and a first conductivity type driftregion formed on a side of the body region closer to the rear surface tobe in contact with the body region, forming a gate trench having asidewall and a bottom wall, passing through the source region and thebody region from the front surface of the semiconductor layer andreaching the drift region, and selectively implanting a secondconductivity type impurity into an edge portion of the gate trench wherethe sidewall and the bottom wall intersect with each other in a partialregion of the gate trench.

The semiconductor device according to the present invention can beproduced by the method.

Preferably, the method for producing a semiconductor device according tothe present invention further includes a step of forming a first regionof a first impurity concentration forming the bottom wall of the gatetrench on the drift region and simultaneously forming a portion of thedrift region other than the first region as a second region of a secondimpurity concentration smaller than the first impurity concentration byimplanting a first conductivity type impurity from the front surface ofthe semiconductor layer after the formation of the gate trench.

Preferably, the step of forming the semiconductor layer includes a stepof stacking the drift region, the body region and the source region inthis order by epitaxy, the step of growing the drift region includes astep of forming a second region of a second impurity concentration fromthe side of the rear surface of the semiconductor layer and forming afirst region of a first impurity concentration greater than the secondimpurity concentration on the second region, and the step of forming thegate trench includes a step of forming the gate trench so that thedeepest portion of the gate trench reaches an intermediate portion inthe thickness direction of the first region.

Preferably, the method for producing a semiconductor device according tothe present invention further includes a step, carried outsimultaneously with the step of forming the gate trench, of forming asource trench having a sidewall and a bottom wall, passing through thesource region and the body region from the front surface of thesemiconductor layer and reaching the drift region, and a step, carriedout simultaneously with the step of implanting the impurity into theedge portion of the gate trench, of selectively implanting a secondconductivity type impurity into an edge portion of the source trenchwhere the sidewall and the bottom wall intersect with each other in apartial region of the source trench.

According to the method, second conductivity type impurity regions canbe simultaneously formed on the edge portion of the gate trench and theedge portion of the source trench respectively as the aforementionedfirst breakdown voltage holding region and the third breakdown voltageholding region. Consequently, a structure for preventing dielectricbreakdown of the gate insulating film can be easily prepared.

The edge portion of the source trench may be a portion where thesidewall and the bottom wall intersect with each other on the wholeregion of the source trench.

The step of forming the semiconductor layer may include a step offorming the body region by performing ion implantation from the frontsurface of the semiconductor layer after forming the semiconductor layerby epitaxy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic plan views of a trench gate MIS transistoraccording to a first embodiment of the present invention, while FIG. 1Ashows a general view and FIG. 1B shows an enlarged internal viewrespectively.

FIG. 2 illustrates sectional views of the trench gate MIS transistorshown in FIGS. 1A and 1B, and shows cutting planes taken along cuttingplane lines A-A and B-B in FIG. 1B respectively.

FIG. 3A is a schematic sectional view showing a part of producing stepsfor the trench gate MIS transistor shown in FIG. 2, and shows cuttingplanes on the same positions as FIG. 2.

FIG. 3B is a diagram showing a step subsequent to FIG. 3A.

FIG. 3C is a diagram showing a step subsequent to FIG. 3B.

FIG. 3D is a diagram showing a step subsequent to FIG. 3C.

FIG. 3E is a diagram showing a step subsequent to FIG. 3D.

FIG. 3F is a diagram showing a step subsequent to FIG. 3E.

FIG. 3G is a diagram showing a step subsequent to FIG. 3F.

FIG. 3H is a diagram showing a step subsequent to FIG. 3G.

FIG. 4 is a schematic sectional view showing a first modification of thetrench gate MIS transistor shown in FIG. 2.

FIGS. 5A and 5B are schematic plan views showing a second modificationof the trench gate MIS transistor shown in FIG. 2, while FIG. 5A shows ageneral view and FIG. 5B shows an enlarged internal view respectively.

FIG. 6 is a sectional view of the trench gate MIS transistor shown inFIGS. 5A and 5B, and shows a cutting plane along a cutting plane lineC-C in FIG. 5B.

FIG. 7 illustrates sectional views of a trench gate MIS transistoraccording to a second embodiment of the present invention, and showscutting planes on the same positions as FIG. 2.

FIG. 8A is a schematic sectional view showing a part of producing stepsfor the trench gate MIS transistor shown in FIG. 7, and shows cuttingplanes on the same positions as FIG. 7.

FIG. 8B is a diagram showing a step subsequent to FIG. 8A.

FIG. 8C is a diagram showing a step subsequent to FIG. 8B.

FIG. 8D is a diagram showing a step subsequent to FIG. 8C.

FIG. 8E is a diagram showing a step subsequent to FIG. 8D.

FIG. 8F is a diagram showing a step subsequent to FIG. 8E.

FIG. 8G is a diagram showing a step subsequent to FIG. 8F.

FIG. 8H is a diagram showing a step subsequent to FIG. 8G.

FIG. 8I is a diagram showing a step subsequent to FIG. 8H.

FIG. 9 illustrates schematic sectional views showing a modification ofthe trench gate MIS transistor shown in FIG. 7.

FIG. 10 illustrates sectional views of a trench gate MIS transistoraccording to a third embodiment of the present invention, and showscutting planes on the same positions as FIG. 2.

FIG. 11A is a schematic sectional view showing a part of producing stepsfor the trench gate MIS transistor shown in FIG. 10, and shows cuttingplanes on the same positions as FIG. 10.

FIG. 11B is a diagram showing a step subsequent to FIG. 11A.

FIG. 11C is a diagram showing a step subsequent to FIG. 11B.

FIG. 11D is a diagram showing a step subsequent to FIG. 11C.

FIG. 11E is a diagram showing a step subsequent to FIG. 11D.

FIG. 11F is a diagram showing a step subsequent to FIG. 11E.

FIG. 11G is a diagram showing a step subsequent to FIG. 11F.

FIG. 11H is a diagram showing a step subsequent to FIG. 11G.

FIG. 12 is a schematic sectional view showing a modification of thetrench gate MIS transistor shown in FIG. 10.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are now described in detail withreference to the attached drawings.

FIGS. 1A and 1B are schematic plan views of a trench gate MIS transistoraccording to a first embodiment of the present invention, while FIG. 1Ashows a general view and FIG. 1B shows an enlarged internal viewrespectively. FIG. 2 illustrates sectional views of the trench gate MIStransistor shown in FIGS. 1A and 1B, and shows cutting planes takenalong cutting plane lines A-A and B-B in FIG. 1B respectively.

An MIS transistor 1 is a trench gate DMISFET (Double diffused MetalInsulator Semiconductor Field Effect Transistor) in which SiC isemployed, and has a chip shape square in plan view as shown in FIG. 1A,for example. The lengths of the chip-shaped MIS transistor 1 in thevertical and horizontal directions on the plane of FIG. 1A are aboutseveral mm respectively.

A source pad 2 is formed on a front surface of the MIS transistor 1. Thesource pad 2 is generally in the form of a square having outwardly bentfour corners in plan view, and formed to generally cover the whole areaof the front surface of the MIS transistor 1. In the source pad 2, aremoval region 3 is formed in the vicinity of the center of one sidethereof. The removal region 3 is a region where no source pad 2 isformed.

A gate pad 4 is arranged on the removal region 3. An interval isprovided between the gate pad 4 and the source pad 2, which areinsulated from each other.

The internal structure of the MIS transistor 1 is now described.

The MIS transistor 1 includes an SiC substrate 5 of an n⁺ type (whoseconcentration is 1×10¹⁸ to 1×10²¹ cm⁻³, for example). The SiC substrate5 functions as a drain of the MIS transistor 1 according to theembodiment, while a front surface 6 (the upper surface) thereof is an Siplane, and a rear surface 7 (the lower surface) thereof is a C plane.

An SiC epitaxial layer 8 of an n⁻ type (whose concentration is 1×10¹⁵ to1×10¹⁷ cm⁻³, for example) lower in concentration than the SiC substrate5 is stacked on the front surface 6 of the SiC substrate 5. Thethickness of the SiC epitaxial layer 8 as a semiconductor layer is 1 μmto 100 μm, for example. The SiC epitaxial layer 8 is stacked on the SiCsubstrate 5 by the so-called epitaxy. The SiC epitaxial layer 8 formedon the front surface 6 which is the Si plane is grown on the Si planeserving as a major growth surface. Therefore, a front surface 9 of theSiC epitaxial layer 8 formed by the growth is an Si plane, similarly tothe front surface 6 of the SiC substrate 5. Alternatively, the frontsurface 9 of the SiC epitaxial layer 8 may be a C plane. If the frontsurface 9 is a C plane, bottom walls 19 (described later) of gatetrenches 15 parallel to the front surface 9 can be formed as C planes.Consequently, the oxidation rate of the bottom walls 19 with respect tosidewalls 18 of the gate trenches 15 can be enlarged, whereby portionsof a gate insulating film 22 present on the bottom walls 19 can bethickened. Therefore, an electric field applied to the gate insulatingfilm 22 on bottom portions of the gate trenches 15 can be relaxed,whereby dielectric breakdown on the bottom portions of the gate trenches15 can be prevented.

The MIS transistor 1 is provided with an active region 10 arranged on acentral portion of the SiC epitaxial layer 8 in plan view to function asthe MIS transistor 1 and a transistor peripheral region 11 surroundingthe active region 10, as shown in FIG. 1A.

In the active region 10, a large number of body regions 12 of a p type(whose concentration is 1.0×10¹⁶ cm⁻³ to 1.0×10¹⁹ cm⁻³, for example) arearrayed on a surface layer portion of the SiC epitaxial layer 8 in theform of a matrix at a constant pitch in a row direction and a columndirection. Each body region 12 is in the form of a square in plan view,and lengths thereof in the vertical and horizontal directions on theplane of FIG. 1B are about 7.2 μm respectively, for example. The bodyregions 12 can be formed by ion implantation as in a step shown in FIG.3A described later, or can be formed by epitaxy. In a case where thesame are formed by ion implantation, a channel length or theconcentration in the body regions 12 can be controlled with excellentin-plane uniformity, whereby stabilization of characteristics andimprovement of a yield can be attained.

On the other hand, a region on a side of the SiC epitaxial layer 8closer to the SiC substrate 5 than the body regions 12 is an n⁻-typedrift region 13 where the state after the epitaxy is maintained.

In the respective body regions 12, source regions 14 of an n⁺ type(whose concentration is 1×10¹⁸ to 1×10²¹ cm⁻³, for example) are formedgenerally on the whole areas thereof on the side of the front surface 9.

The gate trenches 15 passing through the respective source regions 14and the body regions 12 from the front surface 9 of the SiC epitaxiallayer 8 and reaching the drift region 13 are formed in a latticedmanner, to surround the respective body regions 12.

More specifically, each gate trench 15 includes linear portions 16linearly extending between adjacent body regions 12 in the respectiveones of the row direction and the column direction along four sidesurfaces of each body region 12, and intersectional portions 17 wherethe linear portions 16 extending in the row direction and the linearportions 16 extending in the column direction intersect with oneanother. When noting the body regions 12 arrayed in two rows and twocolumns in plan view, the intersectional portions 17 are portions in theform squares in plan view surrounded by inner corners of the fourarrayed body regions 12 and partitioned by extensional lines of foursides of each body region 12. The gate trenches 15 have U-shapedsections where the sidewalls 18 and bottom walls 19 opposed to oneanother are continuous with one another through bent surfaces.

Thus, a large number of unit cells 21 in the form of rectangularparallelepipeds (squares in plan view) each having four corner portions20 are formed on the SiC epitaxial layer 8 in respective window portionssurrounded by the latticed gate trenches 15. In the unit cells 21, thedepth direction of the gate trenches 15 is a gate length direction, anda peripheral direction of the respective unit cells 21 orthogonal to thegate length direction is a gate width direction.

The gate insulating film 22 made of SiO₂ is formed on inner surfaces ofthe gate trenches 15, to cover the whole areas thereof.

In the gate insulating film 22, portions of the gate insulating film 22on the bottom walls 19 are thicker than portions of the gate insulatingfilm 22 on the sidewalls 18, and a top surface thereof is not more thanthe deepest portions of the body regions 12 (the interfaces between thebody regions 12 and the drift region 13), although not strictlyappearing in FIG. 2. More specifically, the thickness of the portions onthe sidewalls 18 is about 1000 Å, while the thickness of the portions onthe bottom walls 19 is 1500 to 3000 Å. Thus, the electric field appliedto the gate insulating film 22 on the bottom portions of the gatetrenches 15 can be relaxed, whereby dielectric breakdown on the bottomportions of the gate trenches 15 can be prevented.

Gate electrodes 23 are embedded in the gate trenches 15 by filling upthe inner side of the gate insulating film 22 with a polysiliconmaterial doped with an n-type impurity in a high concentration. Thus,such a vertical MIS transistor structure is constituted that the sourceregions 14 and the drift region 13 are separately arranged in thevertical direction perpendicular to the front surface 9 of the SiCepitaxial layer 8 through the body regions 12.

Source trenches 24 having square shapes in plan view, passing throughthe respective source regions 14 and the body regions 12 from the frontsurface 9 of the SiC epitaxial layer 8 and reaching the drift region 13are formed on central portions of the respective unit cells 21. Thedepth of the source trenches 24 is identical to that of the gatetrenches 15 according to the embodiment. The source trenches 24 alsohave U-shaped sections where sidewalls 25 and bottom walls 26 opposed toone another are continuous with one another through bent surfaces,similarly to the gate trenches 15.

The SiC epitaxial layer 8 is provided with p-type gate breakdown voltageholding regions 27 and source breakdown voltage holding regions 28 asthird breakdown voltage holding regions formed by implanting a p-typeimpurity into the SiC epitaxial layer 8.

The gate breakdown voltage holding regions 27 are formed along thelatticed gate trenches 15, and integrally include first regions 29 asfirst breakdown voltage holding regions formed on the intersectionportions 17 of the gate trenches 15 and second regions 30 as secondbreakdown voltage holding regions formed on the linear portions 16 ofthe gate trenches 15.

The first regions 29 are formed to reach the body regions 12 immediatelyabove corner edge portions 31 through the bottom walls 19 of the gatetrenches 15 on the intersection portions 17 and the corner edge portions31 of the gate trenches 15 formed under respective corner portions 20 offour unit cells 21 facing each intersection portion 17 from thecorresponding bottom walls 19. In other words, the first regions 29 areprovided in the form of squares slightly larger than the intersectionportions 17 of the gate trenches 15 in plan view, and respective cornersthereof enter the respective corner portions 20 of the four unit cells21 facing each intersection portion 17. The concentration in the firstregions 29 is higher than the concentration in the body regions 12,higher than the concentration in the drift region 13, and 1×10¹⁷ to9×10¹⁹ cm⁻³, for example. A thickness T₁ of the first regions 29 alongthe direction from the bottom surfaces of the gate trenches 15 towardthe SiC substrate 5 is about 0.8 μm, for example.

The second regions 30 are provided in the form of straight lines of aconstant width linking the centers of respective sides of theintersection portions 17 adjacent to one another in plan view, and havea width (1.8 μm, for example) narrower than the width of the linearportions 16 (the distance (1 μm, for example) between side surfaces ofthe gate trenches 15 facing one another). The concentration in thesecond regions 30 is higher than the concentration in the body regions12, higher than that in the first regions 29, and 2×10¹⁷ to 1×10²⁰ cm⁻³,for example. A thickness T₂ of the second regions 30 along the directionfrom the bottom surfaces of the gate trenches 15 toward the SiCsubstrate 5 is smaller than the thickness T₁ of the first regions 29(i.e., T₁>T₂), and about 0.7 μm, for example.

The source breakdown voltage holding regions 28 are formed to reach thebody regions 12 partially forming the sidewalls 25 of the sourcetrenches 24 through the bottom walls 26 of the source trenches 24 andedge portions 32 of the source trenches 24 where the bottom walls 26 andthe sidewalls 25 intersect with one another. The concentration in thesource breakdown voltage holding regions 28 is identical to that in thefirst regions 29 of the gate breakdown voltage holding regions 27(1×10¹⁷ to 9×10¹⁹ cm⁻³, for example). A thickness T₃ of the sourcebreakdown voltage holding regions 28 in the direction from the bottomsurfaces of the source trenches 24 toward the Si substrate 5 isidentical to the thickness T₁ of the first regions 29 of the gatebreakdown voltage holding regions 27 (about 0.8 μm, for example).

On central portions of the bottom walls 26 of the respective sourcetrenches 24, p⁺-type body contact regions 33 (whose concentration is1.0×10¹⁸ cm⁻³ to 2.0×10²¹ cm⁻³, for example) are formed on surface layerportions of the source breakdown voltage holding regions 28.

In the transistor peripheral region 11, a plurality of (according to theembodiment, four) p-type guard rings 34 are formed on the surface layerportion of the SiC epitaxial layer 8 at an interval from the activeregion 10, to surround the unit cells 21 (the active region 10) arrayedin the form of a matrix. The guard rings 34 can be formed through thesame ion implantation step as a step of forming the p-type body regions12.

The respective guard rings 34 are provided in the form of quadrangularrings in plan view along the outer periphery of the MIS transistor 1 inplan view.

An interlayer dielectric film 35 made of SiO₂ is stacked on the SiCepitaxial layer 8, to cover the gate electrodes 23.

Contact holes 36 larger in diameter than the source trenches 24 areformed in the interlayer dielectric film 35 and the gate insulating film22. Thus, the whole of the source trenches 24 (i.e., the sidewalls 25and the bottom walls 26 of the source trenches 24) of the respectiveunit cells 21 and peripheral edge portions of the source trenches 24 onthe front surface 9 of the SiC epitaxial layer 8 are exposed in thecontact holes 36, and steps responsive to the vertical differencebetween the front surface 9 and the bottom walls 26 are formed.

A source electrode 37 is formed on the interlayer dielectric film 35.The source electrode 37 collectively enters the source trenches 24 ofall unit cells 21 through the respective contact holes 36, and is incontact with the body contact regions 33, the source breakdown voltageholding regions 28, the body regions 12 and the source regions 14successively from the side of the bottoms of the source trenches 24 inthe respective unit cells 21. In other words, the source electrode 37serves as a wire common to all unit cells 21.

An interlayer dielectric film (not shown) is formed on the sourceelectrode 37, and the source electrode 37 is electrically connected tothe source pad 2 (see FIG. 1A) through the interlayer dielectric film(not shown). On the other hand, the gate pad 4 (see FIG. 1A) iselectrically connected to the gate electrodes 23 through a gate wire(not shown) drawn on the interlayer dielectric film (not shown).

The source electrode 37 has such a structure that a Ti/TiN layer and anAl layer are stacked successively from the side in contact with the SiCepitaxial layer 8, for example.

A drain electrode 38 is formed on the rear surface 7 of the SiCsubstrate 5, to cover the whole area thereof. The drain electrode 38serves as an electrode common to all unit cells 21. Such a multilayerstructure (Ti/Ni/Au/Ag) that Ti, Ni, Au and Ag are stacked successivelyfrom the side of the SiC substrate 5 can be applied to the drainelectrode 38, for example.

FIGS. 3A to 3H are schematic sectional views showing parts of producingsteps for the trench gate MIS transistor shown in FIG. 2, and showcutting planes on the same positions as FIG. 2.

In order to produce the MIS transistor 1, an SiC crystal is grown on thefront surface 6 (the Si plane) of the SiC substrate 5 by epitaxy such asCVD, LPE or MEB while doping the same with an n-type impurity (N(nitrogen), P (phosphorus), As (arsenic) or the like, for example), asshown in FIG. 3A. Thus, the SiC epitaxial layer 8 of the n⁻ type isformed on the SiC substrate 5.

Then, a p-type impurity (Al (aluminum), B (boron) or the like, forexample) is implanted into the SiC epitaxial layer 8 from the frontsurface 9 of the SiC epitaxial layer 8.

Then, an n-type impurity is implanted into the SiC epitaxial layer 8from the front surface 9 of the SiC epitaxial layer 8.

Then, the SiC epitaxial layer 8 is heat-treated at 1400° C. to 2000° C.,for example. Thus, ions of the p-type impurity and the n-type impurityimplanted into the surface layer portion of the SiC epitaxial layer 8are activated, and the body regions 12, the source regions 14 and theguard rings 34 are simultaneously formed in response to the implantedportions. Further, the drift region 13 maintaining the state after theepitaxy is formed on a base layer portion of the SiC epitaxial layer 8.

Then, the SiC epitaxial layer 8 is etched by employing a mask havingopenings on regions for forming the gate trenches 15 and the sourcetrenches 24, as shown in FIG. 3B. Thus, the SiC epitaxial layer 8 isdry-etched from the surface 9 (the Si plane), and the gate trenches 15and the source trenches 24 are simultaneously formed. Along with this,the large number of unit cells 21 are formed on the SiC epitaxial layer8. As etching gas, mixed gas (SF₆/O₂ gas) containing SF₆ (sulfurhexafluoride) and O₂ (oxygen) or mixed gas (SF₆/O₂/HBr gas) containingSF₆, O₂ and HBr (hydrogen bromide) can be employed, for example.

Then, a first resist 39 having openings exposing the intersectionportions 17 of the gate trenches 15 and the source trenches 24 is formedon the SiC epitaxial layer 8, as shown in FIG. 3C.

Then, a p-type impurity is implanted toward the intersection portions 17of the gate trenches 15 and the source trenches 24 exposed from theopenings of the first resist 39, as shown in FIG. 3D. At this time,neither the sidewalls 18 of the gate trenches 15 (the intersectionportions 17) nor the sidewalls 25 of the source trenches 24 are coveredwith the first resist 39, whereby it follows that the p-type impurity isimplanted also into the sidewalls 18 and 25. Thereafter the SiCepitaxial layer 8 is heat-treated at 1400° C. to 2000° C., for example.Thus, ions of the p-type impurity implanted into the drift region 13 areactivated, and the first regions 29 of the gate breakdown voltageholding regions 27 and the source breakdown voltage holding regions 28are simultaneously formed.

Then, a second resist 40 having openings on regions for forming thesecond regions 30 of the gate breakdown voltage holding regions 27 andthe body contact regions 33 is formed on the SiC epitaxial layer 8, asshown in FIG. 3E. Thus, it follows that the sidewalls 18 and the bottomwalls 19 of the intersection portions 17 of the gate trenches 15 as wellas peripheral edge portions of the sidewalls 25 and the bottom walls 26of the source trenches 24 are covered with the second resist 40.

Then, a p-type impurity is implanted toward the linear portions 16 ofthe gate trenches 15 and central portions of the bottom walls 26 of thesource trenches 24 exposed from the openings of the second resist 40, asshown in FIG. 3F. At this time, the sidewalls 18 and the bottom walls 19as well as the peripheral edge portions of the sidewalls 25 and thebottom walls 25 are protected by the second resist 40, wherebyimplantation of the p-type impurity into these portions can beprevented. Thereafter the SiC epitaxial layer 8 is heat-treated at 1400°C. to 2000° C., for example. Thus, ions of the p-type impurity implantedinto the drift region 13 are activated, and the second regions 30 of thegate breakdown voltage holding regions 27 and the body contact regions33 are simultaneously formed.

Then, an SiO₂ material is deposited from above the SiC epitaxial layer 8by CVD, as shown in FIG. 3G. Thus, the gate insulating film 22 isformed.

Then, a doped polysilicon material is deposited from above the SiCepitaxial layer 8 by CVD. The deposition of the polysilicon material iscontinued at least until the gate trenches 15 and the source trenches 24are filled up. Thereafter the deposited polysilicon material is etchedback until the etched-back surface is flush with the front surface 9 ofthe SiC epitaxial layer 8. Then, only the polysilicon material remainingin the source trenches 24 is removed by dry etching. Thus, the gateelectrodes 23 made of the polysilicon material remaining in the gatetrenches 15 are formed.

Then, an SiO₂ material is deposited from above the SiC epitaxial layer 8by CVD, as shown in FIG. 3H. Thus, the interlayer dielectric film 35 isformed. Then, the interlayer dielectric film 35 and the gate insulatingfilm 22 are continuously patterned by a well-known patterning technique.Thus, the contact holes 36 are formed in the interlayer dielectric film35 and the gate insulating film 22.

Thereafter the source electrodes 37, the drain electrodes 38 etc. areformed, whereby the MIS transistor 1 shown in FIG. 2 is obtained.

In the MIS transistor 1, drain voltage is applied between the source pad2 (the source electrode 37) and the drain electrode 38 (between a sourceand a drain) in a state grounding the source pad 2 (i.e., the sourceelectrode 37 is at 0 V). When voltage of not less than gate thresholdvoltage is applied to the gate pad 4 (the gate electrode 23) in thisstate, channels are formed along the body regions 12 forming thesidewalls of the respective unit cells 21. Thus, current flows from thedrain electrode 38 to the source electrode 37, and the respective unitcells 21 enter ON states.

When the respective unit cells 21 are brought into OFF states (i.e., astate where gate voltage is 0 V) while the voltage is still appliedbetween the source and the drain, on the other hand, an electric fieldis applied to the gate insulating film 22 interposed between the gateelectrode 23 and the SiC epitaxial layer 8.

The electric field results from the potential difference between thegate electrode 23 and the SiC epitaxial layer 8. Equipotential surfacesof extremely high potential are distributed on the bottom walls 19 ofthe gate trenches 15 on the basis (0 V) of the gate electrode 23 and theinterval between the equipotential surfaces is small, whereby anextremely high electric field is caused. When the drain voltage is 900V, for example, equipotential surfaces of 900 V are distributed in thevicinity of the rear surface 7 of the SiC substrate 5 in contact withthe drain electrode 38 and a voltage drop takes place as directed fromthe rear surface 7 of the SiC substrate 5 toward the side of the frontsurface 9 of the SiC epitaxial layer 8, while equipotential surfaces ofabout several 10 V are distributed in the vicinity of the bottom walls19 of the gate trenches 15. Therefore, an extremely high electric fielddirected toward the side of the gate electrode 23 is caused on thebottom walls 19 of the gate trenches 15. Particularly in the case wherethe gate trenches 15 are formed in a latticed manner and the unit cells21 in the form of quadrangular prisms are arrayed on the window portionsof the latticed gate trenches 15 as in the embodiment, dielectricbreakdown of the gate insulating film 22 particularly easily takes placein the vicinity of the corner edge portions 31 of the gate trenches 15formed on the respective corner portions 20 of the unit cells 21.

More specifically, the distance D₁ (see an A-A section in FIG. 2)between the source trenches 24 adjacent to one another on diagonal linesof the intersection portions 17 of the gate trenches 15 is greater ascompared with the distance D₂ (see a B-B section in FIG. 2) between thesource trenches 24 adjacent to one another through the linear portions16 of the gate trenches 15 (for example, D₁ is 1.4 times D₂ according tothe embodiment). Therefore, the equipotential surfaces enter spacesimmediately under the corner edge portions 31 of the gate trenches 15having relatively wide spaces, to result in aggregation of theequipotential surfaces. Consequently, dielectric breakdown of the gateinsulating film 22 particularly easily takes place in the vicinity ofthe corner edge portions 31 of the gate trenches 15.

In the MIS transistor 1 according to the embodiment, therefore, the gatebreakdown voltage holding regions 27 (the first regions 29) are formedon the corner edge portions 31 of the gate trenches 15. Thus, depletionlayers resulting from junction (p-n junction) between the first regions29 and the drift region 13 can be generated in the vicinity of thecorner edge portions 31 of the gate trenches 15. In the MIS transistor1, further, the source breakdown voltage holding regions 28 are formedon the edge portions 32 of the source trenches 24 formed on the centralportions of the respective unit cells 21. Therefore, depletion layersresulting from junction (p-n junction) between the source breakdownvoltage holding regions 28 and the drift region 13 can be spread towardthe corner edge portions 31 of the gate trenches 15 surrounding thesource trenches 24.

The equipotential surfaces can be prevented from entering the spacesbetween the corner edge portions 31 of the gate trenches 15 and the edgeportions 32 of the source trenches 24, and can be separated from thegate insulating film 22, due to the presence of the depletion layers.Consequently, aggregation of the equipotential surfaces in the vicinityof the corner edge portions 31 of the gate trenches 15 can be prevented.Consequently, the electric field applied to the gate insulating film 22can be diminished, whereby dielectric breakdown can be suppressed.Further, the concentration in the first regions 29 is higher than theconcentration in the drift region 13, whereby the depletion layersresulting from the junction (the p-n junction) between the first regions29 and the drift region 13 can be prevented from excessively spreadingin the SiC epitaxial layer 8.

While the first regions 29 are formed to reach the body regions 12immediately above the corner edge portions 31 through the corner edgeportions 31 in the MIS transistor 1, no channels are formed on thecorner portions 20 of the unit cells 21, or the amount of currentflowing through such channels is small even if the same are formed.Therefore, the breakdown preventing effect for the gate insulating film22 can be further improved while hardly influencing the performance ofthe device by forming the gate breakdown voltage holding regions 27 (thefirst regions 29) to reach the portions of the body regions 12immediately above the corner edge portions 31.

On the other hand, the gate breakdown voltage holding regions 27 (thesecond regions 30) of the width smaller than the width of the linearportions 16 are formed on the linear portions 16 of the gate trenches15. Thus, depletion layers resulting from junction (p-n junction)between the second regions 30 and the drift region 13 can be generatedalong the linear portions 16 of the gate trenches 15. Therefore,electric fields formed immediately under the linear portions 16 of thegate trenches 15 can be relaxed by the depletion layers. Consequently,an electric field generated on the gate insulating film 22 can beuniformly relaxed over the whole.

Further, the gate breakdown voltage holding regions 27 (the secondregions 30) are not formed on the side walls 18 of the linear portions16 of the gate trenches 15 (i.e., portions of the unit cells 21 wherechannels are formed). Therefore, it is also possible to preciselycontrol channel characteristics.

In addition, the concentration in the second regions 30 is higher thanthe concentration in the first regions 29, and the thickness T₂ of thesecond regions 30 is smaller than the thickness T₁ of the first regions29 (T₁>T₂), whereby rise of channel resistance can also be prevented.

According to the aforementioned producing method, the gate breakdownvoltage holding regions 27 and the source breakdown voltage holdingregions 28 can be simultaneously formed. Consequently, the structure ofthe MIS transistor 1 for preventing dielectric breakdown of the gateinsulating film 22 can be easily prepared.

FIG. 7 illustrates sectional views of a trench gate MIS transistoraccording to a second embodiment of the present invention, and showscutting planes on the same positions as FIG. 2. Referring to FIG. 7,portions corresponding to the respective portions shown in FIG. 2 aredenoted by reference numerals identical to the reference numeralsassigned to the respective portions, and description is omitted as tothe portions.

While the drift region 13 has been formed only by the low-concentrationregion of the n⁻-type (whose concentration is 1×10¹⁵ to 1×10¹⁷ cm⁻³, forexample) in the aforementioned first embodiment, a drift region 72 of anMIS transistor 71 according to the second embodiment has such astructure that two layers whose impurity concentrations are differentfrom each other are stacked along the thickness direction of an SiCepitaxial layer 8, and includes an n⁻-type low-concentration region 73as an example of a lower layer side second region in contact with afront surface 6 of an SiC substrate 5 and an n-type high-concentrationregion 74 as an example of an upper layer side first region formed onthe low-concentration region 73. The concentration in thelow-concentration region 73 is 1×10¹⁵ to 1×10¹⁷ cm⁻³, for example, andthe concentration in the high-concentration region 74 is 2×10¹⁵ to1×10¹⁸ cm⁻³, for example.

An interface 75 (an upper end of the low-concentration region 73)between the low-concentration region 73 and the high-concentrationregion 74 undulates in response to steps caused by partial lowering of afront surface of the SiC epitaxial layer 8 resulting from formation ofgate trenches 15 and source trenches 24. Thus, the high-concentrationregion 74 on the side of a front surface 9 is so formed as to form thefront surface 9 of the SiC epitaxial layer 8, sidewalls 18 and bottomwalls 19 of the gate trenches 15 and sidewalls 25 and bottom walls 26 ofthe source trenches 24. On the other hand, the low-concentration region73 has low step portions 76 on portions opposed to the bottom walls 19of the gate trenches 15 and the bottom walls 26 of the gate trenches 24and has high step portions 77 on portions opposed to top portions (thefront surface 9) of unit cells 21, for example, in the respective unitcells 21.

The interface 75 so undulates that thicknesses T₄, T₅ and T₆ of thehigh-concentration region 74 along a direction from the respective onesof the top portions (the front surface 9) of the unit cells 21, bottomsurfaces of the gate trenches 15 and bottom surfaces of the sourcetrenches 24 toward the SiC substrate 5 are uniform. The thicknesses T₄,T₅ and T₆ are greater than thicknesses T₁ and T₂ of gate breakdownvoltage holding regions 27 and a thickness T₃ of source breakdownvoltage holding regions 28. Thus, the gate breakdown voltage holdingregions 27 (first regions 29 and second regions 30) and the sourcebreakdown voltage holding regions 28 are covered with thehigh-concentration region 74.

FIGS. 8A to 8I are schematic sectional views showing parts of producingsteps for the trench gate MIS transistor shown in FIG. 7, and showcutting planes on the same positions as FIG. 7.

In order to produce the MIS transistor 71, steps similar to those inFIGS. 3A and 3B are carried out as shown in FIGS. 8A and 8B, whereby then⁻-type SiC epitaxial layer 8 is formed on the SiC substrate 5, and bodyregions 12, source regions 14, guard rings 34 and the drift region 72are simultaneously formed on the SiC epitaxial layer 8. Thereafter theSiC epitaxial layer 8 is dry-etched from the front surface 9 (an Siplane), and the gate trenches 15 and the source trenches 24 aresimultaneously formed.

Then, an n-type impurity is implanted into the SiC epitaxial layer 8over the whole area of the front surface (including respective bottomsurfaces of the gate trenches 15 and the source trenches 24) thereofwithout forming a mask on the front surface 9 of the SiC epitaxial layer8, as shown in FIG. 8C. Then, the SiC epitaxial layer 8 is heat-treatedat 1400° C. to 2000° C., for example. Thus, ions of the n-type impurityimplanted into the SiC epitaxial layer 8 are activated, and thehigh-concentration region 74 is formed on the upper side of the driftregion 72. Further, the low-concentration region 73 maintaining theimpurity concentration in the drift region 72 is formed on the lowerside of the drift region 72.

Thereafter steps similar to those in FIGS. 3C to 3H are carried out asshown in FIGS. 8D to 8I, whereby the MIS transistor 71 shown in FIG. 7is obtained.

As hereinabove described, functions/effects similar to those of theaforementioned MIS transistor 1 can be developed also according to theMIS transistor 71.

In the MIS transistor 71, further, the high-concentration region 74 isso formed as to form the bottom walls 19 of the gate trenches 15 wherethe p-type gate breakdown voltage holding regions 27 are formed and thebottom walls 26 of the source trenches 24 where the p-type sourcebreakdown voltage holding regions 28 are formed, and thehigh-concentration region 74 covers the gate breakdown voltage holdingregions 27 and the source breakdown voltage holding regions 28.

Thus, the high-concentration region 74 can be made to bear p-n junctionbetween the gate breakdown voltage holding regions 27 and the sourcebreakdown voltage holding regions 28 in the drift region 72. Therefore,spreading of depletion layers generated from the p-n junction can besuppressed. Consequently, paths of electrons flowing through channelsare not inhibited by the depletion layers but paths of sufficientmagnitudes can be ensured, whereby increase of on-resistance can beprevented.

The thicknesses T₄, T₅ and T₆ of the high-concentration region 74 may benot more than thicknesses T₁ and T₂ of gate breakdown voltage holdingregions 27 and a thickness T₃ of source breakdown voltage holdingregions 28, as in an MIS transistor 78 of FIG. 9, for example. In thiscase, respective bottom portions of the gate breakdown voltage holdingregions 27 and the source breakdown voltage holding regions 28 projectbeyond interfaces 75 toward the side of a low-concentration region 73,and are covered with the low-concentration region 73. Such a structurecan be formed by performing ion implantation at a time of forming thehigh-concentration region 74 with lower energy than the case of the MIStransistor 71. Therefore, the high-concentration region 74 can be easilyformed with low energy.

FIG. 10 illustrates sectional views of a trench gate MIS transistoraccording to a third embodiment of the present invention, and showscutting planes on the same positions as FIG. 2. Referring to FIG. 10,portions corresponding to the respective portions shown in FIG. 2 aredenoted by reference numerals identical to the reference numeralsassigned to the respective portions, and description is omitted as tothe portions.

While the drift region 13 has been formed by only the low-concentrationregion of the n⁻ type (whose concentration is 1×10¹⁵ to 1×10¹⁷ cm⁻³, forexample) in the aforementioned first embodiment, a drift region 82 of anMIS transistor 81 according to the third embodiment has such a structurethat two layers whose impurity concentrations are different from eachother are stacked along the thickness direction of an SiC epitaxiallayer 8, and includes an n⁻-type low-concentration region 83 as anexample of a lower layer side second region in contact with a frontsurface 6 of an SiC substrate 5 and an n-type high-concentration region84 as an example of an upper layer side first region formed on thelow-concentration region 83, similarly to the second embodiment. Theconcentration in the low-concentration region 83 is 1×10¹⁵ to 1×10¹⁷cm⁻¹, for example, and the concentration in the high-concentrationregion 84 is 2×10¹⁵ to 1×10¹⁸ cm⁻³, for example.

The high-concentration region 84 is so formed as to form a front surface9 of the SiC epitaxial layer 8, sidewalls 18 and bottom walls 19 of gatetrenches 15 and sidewalls 25 and bottom walls 26 of source trenches 24.

An interface 85 (an upper end of the low-concentration region 83)between the low-concentration region 83 and the high-concentrationregion 84 has a constant height along top portions (the front surface 9)of unit cells 21, regardless of steps caused by partial lowering of thefront surface of the SiC epitaxial layer 8 resulting from the formationof the gate trenches 15 and the source trenches 24.

The interface 75 has the constant height, whereby thicknesses T₇, T₈ andT₉ of the high-concentration region 84 along a direction from therespective ones of the top portions (the front surface 9) of the unitcells 21, bottom surfaces of the gate trenches 15 and bottom surfaces ofthe source trenches 24 toward the SiC substrate 5 are so set that T₇ isgreater than T₈ and T₉ (T₇>T₈=T₉). This results from the steps caused bythe partial lowering of the front surface of the SiC epitaxial layer 8.

Further, the thicknesses T₈ and T₉ of the high-concentration region 84are greater than thicknesses T₁ and T₂ of gate breakdown voltage holdingregions 27 and a thickness T₃ of source breakdown voltage holdingregions 28. Thus, the gate breakdown voltage holding regions 27 (firstregions 29 and second regions 30) and the source breakdown voltageholding regions 28 are covered with the high-concentration region 74.

FIGS. 11A to 11H are schematic sectional views showing parts ofproducing steps for the trench gate MIS transistor shown in FIG. 10, andshow cutting planes on the same positions as FIG. 10.

In order to produce the MIS transistor 81, an SiC crystal is grown (theconcentration is 2×10¹⁵ to 1×10¹⁸ cm⁻³, for example) on the frontsurface 6 (an Si plane) of the SiC substrate 5 by epitaxy such as CVD,LPE or MEB while doping the same with an n-type impurity, as shown inFIG. 11A. Thus, the n⁻-type low-concentration region 83 is formed on theSiC substrate 5. Then, the dose is enlarged (2×10¹⁵ to 1×10¹⁹ cm⁻³, forexample), and the SiC crystal is further grown while doping the frontsurface 6 with the n-type impurity. Thus, the high-concentration region84 is formed and the SiC epitaxial layer 8 is formed.

Then, a p-type impurity is implanted into the SiC epitaxial layer 8 fromthe front surface 9 of the SiC epitaxial layer 8, and an n-type impurityis thereafter implanted into the SiC epitaxial layer 8 from the frontsurface 9 of the SiC epitaxial layer 8. Then, the SiC epitaxial layer 8is heat-treated at 1400° C. to 2000° C., for example. Thus, ions of thep-type impurity and the n-type impurity implanted into a surface layerportion of the SiC epitaxial layer 8 are activated, and body regions 12,source regions 14 and guard rings 34 are simultaneously formed inresponse to the implanted portions.

Then, a step similar to that in FIG. 3B is carried out as shown in FIG.11B, whereby the gate trenches 15 and the source trenches 24 whosedeepest portions reach intermediate portions in the thickness directionof the high-concentration region 84 respectively are simultaneouslyformed.

Then, a step similar to that in FIG. 3C is carried out as shown in FIG.11C, whereby a first resist 39 having openings exposing intersectionportions 17 of the gate trenches 15 and the source trenches 24 is formedon the SiC epitaxial layer 8.

Then, steps similar to those in FIGS. 3D to 3F are carried out as shownin FIGS. 11D to 11F, whereby the first regions 29 of the gate breakdownvoltage holding regions 27, the source breakdown voltage holding regions28, the second regions 30 of the gate breakdown voltage holding regions27 and body contact regions 33 whose deepest portions reach intermediateportions in the thickness direction of the high-concentration region 84respectively are formed.

Thereafter steps similar to those in FIGS. 3G to 3H are carried out asshown in FIGS. 11G to 11H, whereby the MIS transistor 81 shown in FIG.10 is obtained.

As hereinabove described, functions/effects similar to those of theaforementioned MIS transistor 1 can be developed also according to theMIS transistor 81.

In the MIS transistor 81, further, the high-concentration region 84 isso formed as to form the bottom walls 19 of the gate trenches 15 wherethe p-type gate breakdown voltage holding regions 27 are formed and thebottom walls 26 of the source trenches 24 where the p-type sourcebreakdown voltage holding regions 28 are formed, and thehigh-concentration region 84 covers the gate breakdown voltage holdingregions 27 and the source breakdown voltage regions 28.

Thus, the high-concentration region 84 can be made to bear p-n junctionbetween the gate breakdown voltage holding regions 27 and the sourcebreakdown voltage holding regions 28 in the drift region 82. Therefore,spreading of depletion layers generated from the p-n junction can besuppressed. Consequently, paths of electrons flowing through channelsare not inhibited by the depletion layers but paths of sufficientmagnitudes can be ensured, whereby increase of on-resistance can beprevented.

The thicknesses T₈ and T₉ of the high-concentration region 84 may be notmore than thicknesses T₁ and T₂ of gate breakdown voltage holdingregions 27 and a thickness T₃ of source breakdown voltage holdingregions 28, as in an MIS transistor 86 of FIG. 12, for example. In thiscase, respective bottom portions of the gate breakdown voltage holdingregions 27 and the source breakdown voltage holding regions 28 projectbeyond interfaces 85 toward the side of a low-concentration region 83,and are covered with the low-concentration region 83.

While the embodiments of the present invention have been described, thepresent invention may be embodied in other ways.

For example, structures inverting the conductivity types of therespective semiconductor portions of the MIS transistors 1, 41, 51, 71,78, 81 and 86 may be employed. For example, the p-type portions may beof the n-type, and the n-type portions may be of the p-type in the MIStransistor 1.

Further, the layers constituting the semiconductor layers in the MIStransistors 1, 41, 51, 71, 78, 81 and 86 are not restricted to theepitaxial layers made of SiC, but may be layers made of a wide bandgapsemiconductor other than SiC, such as GaN (bandgap Eg_(GaN)=about 3.4eV) or diamond (bandgap Eg_(dia)=about 5.5 eV), for example.

The source trenches 24 may be omitted, as in an MIS transistor 41 shownin FIG. 4. In this case, body contact regions 33 can be formed to passthrough source regions 14 and body regions 12 from a front surface 9 ofan SiC epitaxial layer 8 and to reach a drift region 13.

The arrangement mode of the body regions 12 is not restricted to thematrix shape shown in FIG. 1B, but may be a striped shape as in an MIStransistor 51 shown in FIGS. 5A, 5B and 6, for example. In this case,body contact regions 33 may be plurally formed at intervals from oneanother along the longitudinal direction of source trenches 24.

In the MIS transistor 51 shown in FIGS. 5A, 5B and 6, gate breakdownvoltage holding regions 27 are formed along linear gate trenches 15extending between the respective ones of the striped body regions 12.First regions 29 of the gate breakdown voltage holding regions 27 areformed on end portions 52 of the gate trenches 15. On the other hand,second regions 30 of the gate breakdown voltage holding regions 27 areformed on linear portions 53 of the gate trenches 15.

The respective unit cells 21 are not restricted to the shape ofrectangular parallelepipeds (quadrangular prisms), but may be in theform of other polygonal prisms such as triangular prisms, pentagonalprisms or hexagonal prisms, for example.

A semiconductor power device according to the present invention can bebuilt into a power module employed for an inverter circuit constitutinga driving circuit for driving an electric motor utilized as a powersource for an electric car (including a hybrid car), a train or anindustrial robot, for example. The same can also be built into a powermodule employed for an inverter circuit converting power generated by asolar cell, a wind turbine generator or still another power generator(particularly a private power generator) to match with power of acommercial power source.

The embodiments of the present invention are merely specific examplesemployed for clarifying the technical contents of the present invention,and the present invention is not to be interpreted limitedly to thespecific examples, but the spirit and scope of the present invention areto be limited only by the appended claims.

Further, the elements shown in the respective embodiments of the presentinvention can be combined with one another in the range of the presentinvention.

This application corresponds to Japanese Patent Application No.2011-20730 filed with the Japan Patent Office on Feb. 2, 2011, andJapanese Patent Application No. 2011-101786 filed with the Japan PatentOffice on Apr. 28, 2011, the disclosures of which are incorporatedherein by reference.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1 . . . MIS transistor, 8 . . . SiC epitaxial layer, 9 . . .        front surface (of SiC epitaxial layer), 12 . . . body region, 13        . . . drift region, 14 . . . source region, 15 . . . gate        trench, 16 . . . linear portion (of gate trench), 17 . . .        intersection portion (of gate trench), 18 . . . sidewall (of        gate trench), 19 . . . bottom wall (of gate trench), 20 . . .        corner portion (of unit cell), 21 . . . unit cell, 22 . . . gate        insulating film, 23 . . . gate electrode, 24 . . . source        trench, 25 . . . sidewall (of source trench), 26 . . . bottom        wall (of source trench), 27 . . . gate breakdown voltage holding        region, 28 . . . source breakdown voltage holding region, 29 . .        . first region, 30 . . . second region, 31 . . . corner edge        portion (of gate trench), 32 . . . edge portion (of source        trench), 37 . . . source electrode, 38 . . . drain electrode, 41        . . . MIS transistor, 51 . . . MIS transistor, 52 . . . end        portion (of gate trench), 53 . . . linear portion (of gate        trench), 71 . . . MIS transistor, 72 . . . drift region, 73 . .        . low-concentration region, 74 . . . high-concentration region,        75 . . . interface, 78 . . . MIS transistor, 81 . . . MIS        transistor, 82 . . . drift region, 83 . . . low-concentration        region, 84 . . . high-concentration region, 85 . . . interface,        86 . . . MIS transistor

What is claimed is:
 1. A semiconductor device comprising: asemiconductor layer made of a wide bandgap semiconductor having a gatetrench provided with a sidewall and a bottom wall; a gate insulatingfilm formed on the sidewall and the bottom wall of the gate trench; anda gate electrode embedded in the gate trench to be opposed to thesemiconductor layer through the gate insulating film, wherein thesemiconductor layer includes: a source region of a first conductivitytype formed to be exposed on the side of a front surface of thesemiconductor layer for partially forming the sidewall of the gatetrench; a body region of a second conductivity type formed on a side ofthe source region closer to a rear surface of the semiconductor layer tobe in contact with the source region for partially forming the sidewallof the gate trench; a drift region of a first conductivity type formedon a side of the body region closer to the rear surface of thesemiconductor layer to be in contact with the body region for formingthe bottom wall of the gate trench; and a first breakdown voltageholding region of a second conductivity type selectively formed on anedge portion of the gate trench where the sidewall and the bottom wallintersect with each other in a partial region of the gate trench,wherein the drift region includes: a first region of a first impurityconcentration forming the bottom wall of the gate trench; and a secondregion of a second impurity concentration smaller than the firstimpurity concentration formed on a side of the first region closer tothe rear surface of the semiconductor layer to be in contact with thefirst region.
 2. The semiconductor device according to claim 1, whereinthe impurity concentration in the first breakdown voltage holding regionis higher than the impurity concentration in the drift region.
 3. Thesemiconductor device according to claim 1, wherein the gate trench isformed in a latticed manner, the semiconductor layer includes aplurality of unit cells partitioned by the latticed gate trench andprovided in the form of polygonal prisms each having a plurality ofcorner portions, and each unit cell has the source region, the bodyregion and the drift region, and the first breakdown voltage holdingregion is selectively formed on a corner edge portion of the gate trenchformed on the corner portion of the unit cell.
 4. The semiconductordevice according to claim 3, wherein the first breakdown voltage holdingregion is formed to reach a portion of the body region immediately abovethe corner edge portion.
 5. The semiconductor device according to claim3, wherein the first breakdown voltage holding region is selectivelyformed on an intersection portion of the latticed gate trench.
 6. Thesemiconductor device according to claim 3, wherein the semiconductorlayer further includes a second conductivity type second breakdownvoltage holding region, formed on a bottom wall of a linear portion ofthe latticed gate trench, having a width narrower than the width of thelinear portion.
 7. The semiconductor device according to claim 6,wherein the impurity concentration in the second breakdown voltageholding region is higher than the impurity concentration in the firstbreakdown voltage holding region.
 8. The semiconductor device accordingto claim 6, wherein the thickness of the second breakdown voltageholding region is smaller than the thickness of the first breakdownvoltage holding region.
 9. The semiconductor device according to claim1, wherein the semiconductor layer further includes: a source trenchhaving a sidewall and a bottom wall, passing through the source regionand the body region from the front surface and reaching the driftregion; and a second conductivity type third breakdown voltage holdingregion selectively formed on an edge portion of the source trench wherethe sidewall and the bottom wall intersect with each other in a partialregion of the source trench.
 10. The semiconductor device according toclaim 1, wherein the thickness of the first region is greater than thethickness of the first breakdown voltage holding region.
 11. Thesemiconductor device according to claim 1, wherein the thickness of thefirst region is not more than the thickness of the first breakdownvoltage holding region.
 12. The semiconductor device according to claim1, wherein the interface between the first region and the second regionundulates in response to a step caused by partial lowering of the frontsurface of the semiconductor layer resulting from the formation of thegate trench.
 13. The semiconductor device according to claim 1, whereinthe interface between the first region and the second region is at aconstant distance from the front surface of the semiconductor layer. 14.The semiconductor device according to claim 1, wherein a portion of thegate insulating film on the bottom wall of the gate trench is thickerthan a portion of the gate insulating film on the sidewall of the gatetrench, and a top portion thereof is below the deepest portion of thebody region.
 15. The semiconductor device according to claim 1, whereinthe front surface of the semiconductor layer is a C plane.
 16. Thesemiconductor device according to claim 1, wherein the body region isformed by ion implantation.
 17. A semiconductor device comprising: asemiconductor layer made of a wide bandgap semiconductor having a gatetrench provided with a sidewall and a bottom wall; a gate insulatingfilm formed on the sidewall and the bottom wall of the gate trench; anda gate electrode embedded in the gate trench to be opposed to thesemiconductor layer through the gate insulating film, wherein thesemiconductor layer includes: a source region of a first conductivitytype formed to be exposed on the side of a front surface of thesemiconductor layer for partially forming the sidewall of the gatetrench; a body region of a second conductivity type formed on a side ofthe source region closer to a rear surface of the semiconductor layer tobe in contact with the source region for partially forming the sidewallof the gate trench; and a drift region of a first conductivity typeformed on a side of the body region closer to the rear surface of thesemiconductor layer to be in contact with the body region for formingthe bottom wall of the gate trench, wherein the drift region includes: afirst region of a first impurity concentration forming the bottom wallof the gate trench; and a second region of a second impurityconcentration smaller than the first impurity concentration formed on aside of the first region closer to the rear surface of the semiconductorlayer to be in contact with the first region, and the second region ofthe drift region has selectively a projection portion extending to theside of the front surface of the semiconductor layer.
 18. Thesemiconductor device according to claim 17, wherein the semiconductorlayer further includes a first breakdown voltage holding region of asecond conductivity type selectively formed on an edge portion of thegate trench where the sidewall and the bottom wall intersect with eachother in a partial region of the gate trench.
 19. The semiconductordevice according to claim 1, wherein the semiconductor layer is made ofSiC.
 20. The semiconductor device according to claim 17, wherein thesemiconductor layer is made of SiC.